Expanded porous polytetrafluoroethylene products are well known, as for example in U.S. Pat. Nos. 3,953,566 and 3,962,153. The products described therein have air permeabilities on the order of 0.032 metric permeability units (which corresponds to a Frazier number of 0.67). U.S. Pat. No. 4,187,390 describes such products having air permeabilities on the order of 0.6 metric units (which corresponds to a Frazier number of 12). U.S. Pat. No. 4,598,011 also describes such products having air permeabilities expressed as Gurley numbers of 6.5 to 27.5 seconds (which corresponds to Frazier numbers of 0.48 to 0.11).
Heretofore, expanded porous polytetrafluoroethylene membranes with air permeabilities greater than about Frazier numbers of 70 were commercially unknown. Similarly, the production of thin membranes with densities of 0.2 g/cc. or below and porosities at or above 90% and thicknesses below one mil usually led to products which were too fragile to handle.
It is desirable to provide membranes with higher air permeabilities and greater efficiencies. Typically, air filtration to achieve very low particulate contamination in the filtered air has been performed with membranes with small pores and with high efficiencies of particulate capture. Efficiency is a measure of the number of particles of a predefined size which are prevented from penetrating a filter. In the computer disc drive industry, where extreme cleanliness is imperative to success, it is recognized that it is more important to reduce particulate contamination quickly by recirculating the air more rapidly through a more permeable, less efficient filter, and to get to a similar level of total contamination as one would get more slowly with a high efficiency filter. To achieve this end, polytetrafluoroethylene membranes with much higher air permeabilities than have been available heretofore were required. Commercially available porous expanded polytetrafluoroethylene membranes for filtration membranes have air permeabilities of up to a Frazier number of 70 and efficiencies of about 93% when challenged with sodium chloride (0.3 um particle size) aerosols at 10.5 feet per minute face velocity.